1. Field of the Invention
The present invention relates to a steerable antenna array for use with aircraft, satellites and for use with radar systems. More particularly the present invention relates to antennas in which a plurality of antenna elements are mounted onto a curved surface and means is provided for selectively determining the direction in which signals are received and/or transmitted.
2. Description of the Prior Art
It is often desirable to form an assembly of antenna elements in the electrical and geometric arrangement so that the radiation from the arrangement add up to give a maximum field intensity in a particular direction or directions and cancel or substantially cancel in other directions. Each element of such an arrangement or array is an individual radiating element and is structurally separate from the others. The radiation pattern of an array in free space depends on: (1) the geometric arrangement of the individual elements, (2) the relative current phases in each element, (3) the relative magnitude of current in each, and (4) the radiation patterns of the individual elements. It should be added that the gain for a given number of antenna elements is increased by proper spacing of the elements up to a maximum but after the spacing reaches over one wavelength (0.95) this increase generally drops off.
One type of prior art antenna is the conformal phased array antenna. The conformal phased array antenna employs a plurality of antenna elements. The antenna elements are attached to a surface which may be curved at various locations thereof so that each antenna element points at a different elevation and at a different azimuth. In the conformal phased array a beam from such an array can be directed at various angles (between broadside and endfire values) by changing the progressive phase shift along the array. As known, if the phasing is changed continuously, the beam traverses or sweeps angular sectors and such scanning phased array antenna devices find a number of applications. Thus, generally a phase shifter is attached to each of the antenna elements. Depending upon the application, an amplifier is generally attached to each of the phase shifters. If the antenna is used for receiving RF signals, then a low noise amplifier is used; if the antenna is used for sending RF signals, then a high power amplifier is used; and if the antenna is used for both receiving and sending signals, then both a low noise amplifier and a high power amplifier are used. SPMT switches may be employed to render selective ones of the antenna elements effective in the receiving or in the transmitting of RF signals. The SPMT switches provide a means for effectively selecting some of the antenna elements for receiving signals, or for transmitting signals, while rendering other of the antenna elements ineffective.
There are two distinct and formidable disadvantages of conformal phased array antennas. One disadvantage is excessively high cost of fabrication. The excessive cost is inherent in that a separate RF amplifier and a separate phase shifter are required for each antenna element. Another disadvantage is that the conformal phased array antenna is energy inefficient in that RF energy is delivered to each antenna element without regard to whether it faces in the desired direction at a given time.
Another type of prior art antenna is the electronic switched beam array antenna. A switched beam array antenna overcomes the disadvantage of sending RF energy to antenna elements that are not facing in a direction that is useful at a given time. This is accomplished by switching some antenna elements out of the circuit. However, switched beam array antennas also have disadvantages. One disadvantage is that they are generally complex in design because of requiring a separate amplifier for each antenna element. The other disadvantage is that, because of the complexity of the RF power divider, switched beam array antennas are generally limited to frequencies below the S-band.
In one design of an electronic switched beam array antenna which operates at 1.5 GHz, forty-six antenna elements were connected to forty-six separate RF amplifiers. The RF network that connects each antenna element to a respective one of the amplifiers is generally bulky, labor-intensive, and difficult to manufacture. In yet another design, one hundred twenty antenna elements were employed along with one hundred twenty amplifiers. In addition to being expensive to manufacture, the RF feed network which divides the RF signal for each amplifier encountered significant energy losses above 2.0 GHz. In the aforementioned antenna arrays it was found that there was yet another significant problem that was due to beam crossing found in these prior types of arrangement.